The present invention relates to the field of flexible printed wiring boards, particularly to the field of flexible printed wiring boards of multilayer structure.
Recently, flexible wiring boards of multilayer structure are used in many electronic circuits.
As an example, a process for manufacturing a multilayer flexible printed wiring board is explained. Referring to FIG. 20(a), the reference number 311 represents a copper foil having a thickness of dozens of micrometers.
A polyimide varnish is first applied on this copper foil 311 to form a base film 312 consisting of a polyimide film (FIG. 20(b)). Then, a resist layer 313 is formed on base film 312 (FIG. 20(c)), and resist layer 313 is patterned via photographic processes. The reference number 331 in FIG. 20(d) represents an opening in resist layer 313, and base film 312 is exposed at the bottom of this opening 331.
Then, the part of base film 312 exposed at the bottom of opening 331 is etched off (FIG. 20(e)). Then, resist layer 313 is removed to give a patterned base film 312 (FIG. 20(f)).
In FIG. 21(g), base film 312 is inverted with copper foil 311 upward.
A masking film 317 is applied on base film 312 (FIG. 21(h)), and a resist layer 315 is formed on copper foil 311 (FIG. 21(i)).
Then, resist layer 315 is patterned via exposure and development processes. The reference number 332 in FIG. 21(j) represents an opening formed by patterning in resist layer 315. Copper foil 311 is exposed at the bottom of this opening 332.
Then, copper foil 311 at the bottom of opening 332 is etched to pattern copper foil 311 into a first wiring layer 316 (FIG. 21(k)). The reference number 333 represents the part from which copper foil 311 has been removed and an opening segmenting first wiring layer 316. The top of base film 312 is exposed at the bottom of opening 333.
Resist layer 315 is removed (FIG. 21(l)) and a polyimide varnish is applied on the top of first wiring layer 316 so that the polyimide varnish flows into opening 333 in first wiring layer 316 to form a cover film 318 consisting of a polyimide film having a flat surface.
A resist layer 319 is formed on the top of cover film 318 (FIG. 22(n)) and resist layer 319 is patterned via exposure and development processes.
The reference number 334 in FIG. 22(o) represents an opening formed by patterning in resist layer 319. Cover film 318 is exposed at the bottom of this opening 334.
Then, the part of cover film 318 located at the bottom of opening 334 is etched off with a metallic etching solution to pattern cover film 318 so that first wiring layer 316 is exposed at the bottom of the opening 334. The etching solution used here is selected not to etch first wiring layer 316.
Finally, resist layer 319 is removed and followed by heat treatment to imidate base film 312 and cover film 318, whereby a first single-wiring layer board piece 310 is obtained (FIG. 22(q)).
Thus obtained first single-wiring layer board piece 310 comprises first wiring layer 316, patterned base film 312 provided on one side of first wiring layer 316 and patterned cover film 318 provided on the opposite side of first wiring layer 316. Opening 333 in first wiring layer 316 is filled with cover film 318.
The reference number 380 in FIG. 23(a) represents a second single-wiring layer board piece to be laminated to first single-wiring layer board piece 310. This second single-wiring layer board piece 380 comprises a base film 381 consisting of a polyimlde film, a second wiring layer 386 provided on said base film 381 and a cover film 382 provided on said second wiring layer 386.
Said second wiring layer 386 consists of a patterned copper foil and said cover film 382 consists of a polyimide film.
Second single-wiring layer board piece 380 has a plurality of bumps 384 connected to second wiring layer 386 at the bottoms and projecting from cover film 382 at the tops.
First single-wiring layer board piece 310 is opposed to the plane of second single-wiring layer board piece 380 from which the tops of bumps 384 project in parallel thereto, and bumps 384 are aligned with openings 331 in base film 312 to bring bumps 384 into contact with the surface of first wiring layer 316, whereby first and second wiring layers 316 and 386 are connected via bumps 384.
If either one of two cover films 312, 382 includes of a thermoplastic resin having the property of developing adhesiveness upon heating, first and second single-wiring layer board pieces 310, 380 can be bonded together by heating them while bumps 384 are in contact with the surface of first wiring layer 316. The reference number 351 in FIG. 23(b) represents a multilayer wiring board comprising first and second single-wiring layer board pieces 310, 380 bonded together.
The process for forming an opening by patterning a polyimide film by etching as described above provides finer openings than laser etching or drilling so that it is widely used in the manufacture of high-density multilayer flexible wiring boards in which openings should be provided with narrow gaps.
However, the etching process using an alkali solution as described above involves complex control of the temperature or state of the solution. Particularly when etching conditions are insufficiently controlled, variation may occur in the size of openings formed in polyimide.
Moreover, the use of a resist layer consisting of a photosensitive film for forming an opening adds production costs.
An object of the present invention is to simplify the complex conventional process for manufacturing a multilayer wiring board as described above and to provide a single-layer flexible wiring board suitable for preparing a multilayer flexible wiring board, the resulting multilayer flexible wiring board, a process for manufacturing a multilayer flexible wiring board and an ultrasonic manufacturing apparatus suitable for use in the manufacturing process.
In order to attain the above object, the present invention provides a process for manufacturing a multilayer flexible wiring board by using a first single-wiring layer board piece having a first patterned wiring layer and a first resin film in close contact with said first wiring layer, and a second single-wiring layer board piece having a second patterned wiring layer and a plurality of bumps connected to said second wiring layer at the bottoms to laminate said first and second single-wiring layer board pieces into a multilayer flexible wiring board, said process comprising bringing the top of each of said bumps into contact with said first resin film, applying ultrasonic wave to at least one of said first and second single-wiring layer board pieces to force into said first resin film in contact with said each bump to form an opening, and bringing said each bump into contact with said first wiring layer to electrically connect said first and second wiring layers via said each bump.
According to this aspect of the present invention, said each bump may be ultrasonically vibrated in the direction along the surface of said first resin film.
According to the present invention, said application of ultrasonic wave may be continued after the top of said each bump comes into contact with said first wiring layer to ultrasonically bond said each bump to said first wiring layer.
According to the present invention, said first and second wiring layers and said bumps may consist of a metal material based on copper, and either one or both of the surface of at least the top of said each bump or the surface of said first wiring layer in contact with at least the top of said each bump may be coated with a metal material based on one or more metals selected from gold, silver, platinum, palladium, tin, zinc, lead, nickel or iridium.
According to the present invention, said application of ultrasonic wave may be carried out under pressure.
According to the present invention, said first resin film may include a thermosetting resin and may be precured before an opening is formed by said each bump.
According to the present invention, said first resin film may include a thermosetting polyimide film.
According to the present invention, said each bump may be brought into contact with said first resin film to apply ultrasonic wave after a second resin film is provided on the side of said second wiring layer having said bumps in such a manner that said second resin film is in close contact with said second wiring layer and the top of said each bump projects above said second resin film.
According to the present invention, at least the surface of said second resin film may include a resin developing adhesiveness upon heating.
According to the present invention, said second resin film may be heated during said application of ultrasonic wave.
According to the present invention, at least the surface of said second resin film may consist of a thermoplastic polyimide film.
According to the present invention, said each bump may have a size expressed as the sectional area parallel to said second wiring layer of 19.6xc3x9710xe2x88x928 m2 or less at maximum.
The present invention also provides a process for manufacturing a multilayer flexible wiring board by using a first single-wiring layer board piece having a first patterned wiring layer and a first resin film in close contact with said first wiring layer, and a second single-wiring layer board piece having a second patterned wiring layer and a plurality of bumps connected to said second wiring layer at the bottoms to laminate said first and second single-wiring layer board pieces into a multilayer flexible wiring board, said process comprising bringing a projection on an ultrasonic manufacturing apparatus into contact with said first resin film, applying ultrasonic wave to said projection to force into said first resin film by said projection to form an opening, and then bringing the top of each of said bumps of said second single-wiring layer board piece into contact with said first wiring layer at the bottom of said opening.
According to this aspect of the present invention, said first wiring layer may be exposed at the bottom of said opening.
According to the present invention, said ultrasonic manufacturing apparatus may have a plurality of said projections to form a plurality of said openings in said first resin film by a single application of ultrasonic wave.
According to the present invention, said each projection may be ultrasonically vibrated in the direction along the surface of said first resin film.
According to the present invention, said first resin film may be formed by applying a liquid raw marterial on said first wiring layer and curing it by heating, and said opening may be formed in said first resin film in a cured state.
According to the present invention, an adhesive film developing adhesiveness upon heating may be applied after said opening is formed, and said first and second single-wiring layer board pieces may be bonded together via said adhesive film.
The present invention also provides a multilayer flexible wiring board comprising first and second patterned wiring layers, a first resin film interposed between said first and second wiring layers, and a bump connected to said second wiring layer at the bottom, wherein said first resin film has an opening formed by applying ultrasonic wave to said bump to force into it and said bump is left in said opening to electrically connect the top of said bump to said first wiring layer.
According to this aspect of the present invention, a plurality of said openings may be provided and said bump may be left in said each opening.
According to the present invention, said first resin film may include a resin developing adhesiveness upon heating.
According to the present invention, the top of said each bump and said first wiring layer may be ultrasonically bonded to each other.
According to the present invention, the surface of the top of said each bump or the surface of said first wiring layer to be connected to the top of said each bump may be coated with a metal material based on one or more metals selected from gold, silver, platinum, palladium, tin, zinc, lead, nickel or iridium.
The present invention also provides a multilayer flexible wiring board comprising first and second patterned wiring layers, a first resin film interposed between said first and second wiring layers, and a plurality of bumps connected to said second wiring layer at the bottoms, wherein said first rein film has a plurality of openings formed by applying ultrasonic wave to a projection of an ultrasonic manufacturing apparatus to force into it and each of said bumps is located in each of said openings to electrically connect the top of said each bump to said first wiring layer.
According to this aspect of the present invention, said each opening may have an area of 19.6xc3x9710xe2x88x928 m2 or less.
The present invention also provides an ultrasonic manufacturing apparatus comprising an ultrasonic wave generator generating ultrasonic vibration and a resonator transmitting said ultrasonic vibration, wherein said resonator has a plurality of projections capable of simultaneously coming into contact with a flat surface of a work.
According to this aspect of the present invention, an ultrasonic wave vibrating in the direction parallel to said flat surface of said work may be applied to said each projection.
According to the present invention, said each projection may have a size expressed as the cross sectional area parallel to said second wiring layer of 19.6xc3x9710xe2x88x928 m2 or less at maximum. When a shape of the bump having a size as cross sectional area parallel to said second wiring board of 19.6xc3x9710xe2x88x928 m2, or a shape of the opening having same size as said bump is circle, for example, the diameter of circle is 5xc3x9710xe2x88x924 m or less. When the diameter of the projection formed semisphere is 5xc3x9710xe2x88x924 m or less, the projection height is 2.5xc3x9710xe2x88x924 m or less. Therefor, the H1 of bump height and projection height are 2.5xc3x9710xe2x88x924 m or less.
According to the present invention, said ultrasonic wave generator may be oblique to said flat surface of said work.
When said ultrasonic manufacturing apparatus is used to form a plurality of openings in a first single-wiring layer board piece having a first patterned wiring layer and a first resin film in close contact with said first wiring layer, said each projection may be provided at the location corresponding to the location of each bump on a second single-wiring layer board piece to be bonded to said first single-wiring layer board piece.
According to this embodiment of the present invention, said resonator may be replaceable.
The present invention also provides an ultrasonic manufacturing apparatus comprising an ultrasonic wave generator generating ultrasonic vibration and a resonator transmitting said ultrasonic vibration, wherein said resonator has a pressing face to be pressed against a flat surface of a work and said resonator is oblique to said flat surface of said work when said pressing face is pressed against said flat surface of said work.